1. Field of the Invention
The present invention relates to a wave gear drive, or more in particular, to a wave gear drive having a transfer-torque control mechanism that prevents excessive torque transfer.
2. Prior Art Description
It is well known that an actuator built in a robot arm or the like employs a wave gear drive as a reduction mechanism for reducing the rotational speed of a motor serving as a drive source.
The wave gear drive basically includes an annular rigid internal gear, an annular flexible external gear arranged inside of the annular rigid internal gear, and a wave generator fitted inside of the annular flexible external gear. The wave generator has an outline of, for example, an elliptical shape, and urges the flexible external gear radially so as to be brought into partial engagement with the rigid internal gear. When the wave generator is connected to the high-speed rotation input shaft such as the motor and rotates at a high speed, the engaging points of both two gears move along the circumferential direction. As a result, a relative rotational motion corresponding to the difference in the number of teeth of both gears is generated therebetween. As the rigid internal gear is generally fixed, the output of reduced rotational speed is derived from the flexible external gear.
When a member at a driven side such as a robot arm collides with other member during operation, the torque generated by the collision is transferred in the reverse direction through the power transmission path from the motor to the driven member via the wave gear drive or the like. The turning force including the input side inertia of the motor or the like acts on each member constituting the power transmission path, thus generating excessive torque. As a result, various problems resulting from the excessive torque may occur, for example, the portion constituting the flexible external gear of the wave gear drive is damaged.
A method for preventing the aforementioned problem has been generally taken by mounting a transfer-torque control mechanism on such portion as the motor output shaft so as to prevent the transfer of the torque exceeding the limited value.
Mounting the aforementioned transfer-torque control mechanism, however, may increase the number of parts constituting the power transmission path, thus requiring a larger installation space as well as increasing the cost.
It is an object of the present invention to propose a wave gear drive with a compact transfer-torque control mechanism built therein for preventing the aforementioned problems.
In order to achieve the above-described and other objects, according to the present invention, there is provided a wave gear drive including an annular rigid internal gear, an annular flexible external gear arranged inside of the annular rigid internal gear, and a wave generator fitted inside of the annular flexible external gear, in which the wave generator urges said flexible external gear radially so as to be partially in mesh with the rigid internal gear, and moves points of partial engagement in the circumferential direction so as to generate a relative rotation between the rigid internal gear and the flexible external gear corresponding to the difference in the number of teeth therebetween, characterized in that the wave generator includes a rigid cam plate having a predetermined outline and a hub inserted into a fitting hole formed in the center of the rigid cam plate; and the wave generator further includes a transfer-torque control mechanism for limiting a torque transferred to the flexible external gear through the wave generator, the transfer-torque control mechanism being configured to fasten the rigid cam plate and the hub so as to generate a slip in a rotational direction by transferring the torque exceeding a predetermined value.
According to a preferred embodiment, the wave generator includes an outer annular hub with an Oldham""s mechanism fitted in the fitting hole formed in the center of the rigid cam plate and an inner annular hub inserted inside of the outer annular hub, both of which constitute the hub; and the transfer-torque control mechanism includes an annular flange radially expanding outward and formed at one end of the inner annular hub in an axial direction, an external thread formed on the outer peripheral surface of the other end of the inner annular hub in the axial direction, and a nut having an internal thread formed on the inner peripheral surface so as to be screwed with the external thread, and the outer annular hub is urged against the annular flange by the nut.
According to another preferred embodiment, the transfer-torque control mechanism includes an annular flange radially expanding outward and formed at one end of the hub in an axial direction, an external thread formed on the outer peripheral surface of the other end of the annular hub in the axial direction, and a nut having an internal thread formed on the inner peripheral surface so as to be screwed with the external thread, and the rigid cam plate is urged against the annular flange by the nut.
In this embodiment, the hub can be integrated with an input shaft connected thereto.
Also the external thread and the nut can be replaced by an axially extending bolt hole formed in the end surface of the input shaft and a fastening bolt adapted to be screwed into the bolt hole, and the rigid cam plate is urged against the annular flange by the fastening bolt.
According to another embodiment of the invention, the wave generator is characterized in that the wave generator includes an outer annular hub with an Oldham""s mechanism fitted in a fitting hole formed in the center of the rigid cam plate and an inner annular hub inserted inside of the outer annular hub, both of which constitute the hub; and the transfer-torque control mechanism includes an engaging hole formed in the outer peripheral surface of the inner annular hub, a radially extending through hole formed in the outer annular hub, a ball inserted into the through hole to be engaged with the engaging hole, and an elastic member that urges the ball radially inward.
Another embodiment of the invention is characterized in that the transfer-torque control mechanism includes an engaging hole formed in the outer peripheral surface of the hub, a radially extending through hole formed in the rigid cam plate, a ball inserted into the through hole to be engaged with the engaging hole, and an elastic member that urges the ball radially inward.
According to still another embodiment of the invention, the transfer-torque control mechanism includes an engaging hole formed in the inner peripheral surface of a fitting hole of the rigid cam plate, a radially extending through hole formed in the hub, a ball inserted into the through hole to be engaged with the engaging hole, and an elastic member that urges the ball radially outward.
According to yet another embodiment of the present invention, the wave generator includes an outer annular hub with an Oldham""s mechanism fitted in a fitting hole formed in the center of the rigid cam plate, and an inner annular hub inserted inside of the outer annular hub, both of which constitute the hub; and the transfer-torque control mechanism includes an annular flange radially expanding outward and formed at one end of the inner annular hub in an axial direction, an engaging hole formed in the annular end surface of the annular flange, an axially extending through hole formed in the outer annular flange, a ball inserted into the through hole to be engaged with the engaging hole, and an elastic member that urges the ball against the engaging hole in the axial direction.
According to another embodiment of the invention, the wave generator includes an outer annular hub with an Oldham""s mechanism fitted in a fitting hole formed in the center of the rigid cam plate, and an inner annular hub inserted inside of the outer annular hub, both of which constitute the hub; and the transfer-torque control mechanism includes an engaging hole formed in the outer peripheral surface of the inner annular hub, an axially extending through hole formed in the outer annular flange, a ball inserted into the through hole to be engaged with the engaging hole, and an elastic member that urges the ball against the engaging hole.
In this embodiment, the inner annular hub can be integrated with an input shaft connected thereto.
Also, the outer annular hub and the rigid cam plate can be formed as the integrated portion.
Next, according to still another embodiment of the invention, the transfer-torque control mechanism includes an engaging hole formed in the outer peripheral surface of the hub, a radially extending through hole formed in the rigid cam plate, a roller inserted into the through hole to be engaged with the engaging hole, and an elastic member that urges the roller radially inward; and the engagement between the axial end surfaces of the engaging hole and the end surfaces of the roller prevents displacement of the rigid cam and the annular hub in the axial direction.
According to another embodiment of the invention, the transfer-torque control mechanism includes at least one pin inserted into the rigid cam plate and the hub, and a breaking torque of the pin is set to assume a predetermined value.
In this embodiment, the hub can be integrated with an input shaft connected thereto.